What to do when an error happens. By default, debug, meaning to
enter the debugger. Other values include backtrace, to show a
backtrace without entering the debugger, or report, to simply
show a short error printout.

Default values for REPL options may be set using
repl-default-option-set! from (system repl common):

Scheme Procedure: repl-default-option-set!key value

Set the default value of a REPL option. This function is particularly
useful in a user’s init file. See Init File.